The present invention relates to magnetic bearings for accepting axial thrust loads.
In an axial magnetic bearing the configuration is generally such that a solid, circular ferromagnetic disc., secured to a shaft, is used for the rotor of the bearing. Annular electromagnetic structures are generally sited on each side of an adjacent the disc and, depending upon the thrust direction, one or other of the electromagnets structure balances out thrust forces along the shaft axis. This is achieved by a control system which varies the current to the electromagnet structure in order to maintain a substantially constant gap between the magnet and disc face.
The diameter of the disc has generally been dictated by the magnitude of the axial load which it is necessary to counteract.
Where the anticipated rotational speed of the shaft and disc is very high and is also allied with high thrust loads the stresses generated in the rotating disc may exceed the mechanical strength of the ferromagnetic material from which it is made.
It is an object of the present invention, therefore, to provide an axial magnetic bearing assembly in which, in operation, there are lower induced stresses within each provided thrust bearing member, than for a conventional axial magnetic bearing assembly capable of accepting the same anticipated maximum thrust.
It is another object of the present invention to provide an axial magnetic bearing assembly extending less in a radial direction than a conventional axial magnetic bearing assembly capable of accepting the same anticipated maximum thrust.
According to the present invention a magnetic bearing assembly has a rotatable arrangement, at least including a shaft, and, possibly, one, or two additional parts secured to the shaft, ferromagnetic material of the rotatable arrangement both provides, in relation to the shaft axis, two axially spaced, and radially extending, faces to accept thrusts along the shaft in one direction, and extends completely between the two thrust faces, and the assembly also has a magnetic circuit comprising the ferromagnetic material between the two thrust faces, and an electromagnet, the electromagnet has an annular coil and an annular, ferromagnetic core, both the coil and the core being coaxial with the shaft, the coil is located in a channel provided in the core surface opposite to the shaft, and each pole has a radially extending face opposite to, and spaced from, an individually associated thrust face of the two radially extending thrust faces.
Whether the shaft comprises the sole, or a constituent, part of the rotatable arrangement, it may be of ferromagnetic material. When the shaft is of ferromagnetic material, each thrust face may be provided, individually, by a thrust collar, either the thrust collars being machined from the shaft material, or initially separate thrust collars of ferromagnetic material being secured to the shaft.
Preferably ferromagnetic material providing at least one of the thrust faces is tapered in the radially outward direction to reduce the mass thereof.
Preferably at least one of the poles of the electromagnet structure is tapered in the radially inward direction to minimise flux leakage into the shaft.
It has been found with axial magnetic bearings according to the present invention that the form of each electromagnet structure used allows the use of larger coil windings than is conventional in such an assembly. This gives rise to lower power or current consumption leading to correspondingly lower heat dissipation requirements.